Conventional compression coding (hereinafter, simply referred to as “coding”) techniques using differences between pictures include coding techniques defined by the MPEG (Moving Picture Experts Group) (See Non Patent Literature 1). Recently, the H.264 standards (See Non patent Literature 2) for achieving higher compression have been employed as coding standards. As with the conventional MPEG standards, in H.264, coding and decoding are performed in units of a rectangular block called macroblock composed of 16×16 pixels.
As with the conventional MPEG standards, a coding algorithm called inter-picture prediction is used in H.264. In the case of decoding a macroblock on which inter-picture prediction processing needs to be performed, its pixel values are predicted using information of decoded picture(s). Therefore, data of decoded pictures are kept, and referred to as necessary. In contrast, in the case of decoding a macroblock on which intra-picture prediction needs to be performed, the macroblock is decoded using information of a macroblock which immediately precedes a currently being decoded macroblock and information of its adjacent macroblocks each of which is spatially located at the left, top, left top or right top position. Therefore, information of decoded macroblocks within the same picture is kept, and used for decoding the subsequent macroblocks.
Some of the techniques disclose decoding a bit stream coded using coding methods such as the MPEG and H.264 standards as mentioned above by segmenting the bit stream into plural bit streams, and making plural decoding chips decode the respective segmented bit streams. At this time, the decoding chips must mutually transfer the decoded data in order to decode the bit streams, using information of decoded pictures or decoded macroblocks.
For example, Patent Literature 1 has proposed a parallel decoding processing approach performed by a system which segments a bit stream into plural bit streams and decodes the bit streams by plural decoding circuits while mutually transferring data necessary for inter-picture prediction via a bus.
FIG. 12 is a diagram showing the decoding order in a picture that is decoded in parallel by the plural decoding circuits disclosed in Patent Literature 1. In the example shown in the diagram, the picture is segmented into two areas that are a top area and a bottom area, and each of the two areas is decoded by a corresponding one of the first and second decoding circuits. Here, as shown in FIG. 12, the technique disclosed in Patent Literature 1 decodes macroblock lines in a picture sequentially from bottom, whereas, generally, macroblock lines in a picture are decoded sequentially from top.
This makes it possible to prioritize decoding of data that is referred to at the time when the second decoding circuit decodes a next picture, over decoding of data that is not referred to. Accordingly, it is possible to reduce time for waiting data transfer from the first decoding circuit when the second decoding circuit decodes a next picture.
On the other hand, Patent Literature 2 discloses a technique for monitoring a current processing status of macroblocks in a picture, and for processing an object macroblock to be coded in the case where reference macroblocks for the object macroblock have been processed. For example, in the case of performing intra-picture prediction processing, parallel processing is performed by detecting completion of processing of adjacent macroblocks each of which is spatially located at the left, top, left top, or right top position, and assigning each of the macroblocks having complete reference data to a corresponding one of the plural circuits.